Pyroclastic processes and materials

Pyroclastic means 'fire broken' and is the term for rocks formed from fragments produced by volcanic explosions.

Pyroclastic processes

Magmas with explosive volatile contents can vary in styles of eruption. The reduction in pressure as the magma approaches the surface drives out bubbles of dissolved gases in bubbles and expands the volume of the magma. This degassing magma is driven up into high eruption clouds and also hugs the ground as pyroclastic flows, clouds of magma droplets, ash clouds and gases. Gases from such eruptions can be deadly. They include water vapour, carbon dioxide, carbon monoxide, sulfur dioxide, hydrogen sulfide and hydrofluoric acid.

Case study of an eruption

Ruapehu volcano on the North Island of New Zealand has a vent-related hydrothermal system and is covered by a 10 000 000 m3 acidic crater lake where volcanic gases accumulate. The greatest concentration of harmful gases occurs during early water-charged magmatic eruptions. The volcanic ash from these eruptions is six times more concentrated in fluorine than the original magma. Much of this fluorine forms slowly from soluble inorganic compounds which release fluorine to the soil over a long time. However, this fluorine-bearing ash is soluble in the acidic digestion system of animals. This led to the death of several thousand sheep from fluorosis during the 1995-1996 eruption of Ruapehu. This is an example f how volcanic gases released during eruptions can create both short-term and long-term environmental hazards.

Pyroclastic materials

The pyroclastic material erupted from an explosive volcanic eruption may be ejected as fragments, resulting in scoria cones or ash fall deposits, or it may spread outwards in ash flow deposits. This fragmental material is classified on the basis of grain size.

Scoria cones or cinder cones

Large blocks of lava (bombs) cannot be ejected far from a volcanic vent because of their weight. Eruptions that eject large amounts of bombs or lapilli (little stones) build up scoria cones or cinder cones.

Lava tubes

These form when the outer surface of a lava flow hardens but the inside remains hot and lava continues to flow through. Then, when the eruption wanes, most of the lava drains out forming a hollow lava tube. Some of the largest and most spectacular lava tubes on Earth are found at Undara in northern Queensland.

Ash falls

The finer materials of explosive volcanic eruptions are ejected into the air, eventually falling to the ground as a layer of ash. The extent of the ash fall depends upon the height that the ash is ejected along with the speed and direction of the prevailing wind at the time. Most ash clouds are only ejected 1 km into the air and fall back onto the volcanic cone. Very powerful eruptions may hurl ash over 10 km into the air. These are relatively rare and occur only a few times every 100 years. When the ash from these eruptions covers a large area, they form sheet deposits. Because ash falls consist of relatively loose material, many deposits rapidly erode and are washed elsewhere. In the 79 AD eruption of Vesuvius, which partly destroyed the Roman town of Pompeii, the eruption plume rose to a great height and covered the surrounding area with pumice and hot ash to an extent of 450 km2. As the ash cloud collapsed, the ash suffocated most of the victims.

Some ash falls are enormous in extent. The 1783 Laki fissure eruption in Iceland covered the entire island in ash, completely devastating the vegetation, and sent a dust cloud over Europe and North America for several months. The 1883 eruption of Krakatoa threw ash so high into the atmosphere that the finer dust eventually spread all around the Earth in just 14 days.

Ash flows

Many of the world's large accumulations of rhyolitic and dacitic debris are ash flows. This volatile-rich material can travel over great distances at fast speed and these flows also called nuée ardente (French for fiery cloud). They often occur as avalanches when lava domes collapse or volcanic slopes shake. In the 186 AD eruption of Taupo in New Zealand, an ash flow of 30 km3 of material was ejected in under ten minutes and flowed for 80 km in all directions at a speed of 300 m/s, even flowing straight over mountains in its path.

Lahars

Lahars are mud flows composed of pyroclastic material. They occur where the ground has a slope and water is present. In tropical areas of high rainfall, thick ash deposits become water-logged during the wet season, becoming unstable on the steep slopes, and trigger mud flows. In high areas, mud flows can result from the melting of ice and snow surrounding volcanic summits. Sometimes water in a crater lake may be discharged when the volcano becomes active again. Lahars were named from the islands of Indonesia where they cause loss of many lives.